Studies on Huntington disease (HD), also characterized by a toxic gain of function, reported a similar protective role of wild‐type huntingtin (Htt) protein in the disease: the absence of wild‐type protein in HD mutant mouse and Drosophila models of Htt toxicity leads to a more severe phenotype (Van Raamsdonk et al, 2005; Zhang et al, 2009). This evidence concerns the gene HTT and juvenile Huntington disease.